1. Field of the Invention
The present invention relates to a frequency modulation apparatus and a frequency modulation method, which generate an image clock used for an on/off control of a laser beam scanning on a image bearing body such as a photosensitive drum and the like.
2. Related Background Art
In general, in an image forming apparatus of an  electro-photographic system, while laser light emitted from a semiconductor laser is turned on and off, this laser light is scanned by a polygonal rotating mirror (polygon mirror) so as to irradiate a photosensitive body, thereby performing a latent image formation.
In such an image forming apparatus, an image clock of a constant frequency is used for an on/off control of the laser light. The reason why is because, if the frequency of this image clock is not constant, the on/off timing of the laser light deviates from a normal timing, and thereby a dot forming position of an electrostatic latent image formed on the photosensitive body is subtly displaced, and this results in the occurrence of an image distortion, a color drift and a color shading.
Further, between the polygonal mirror and the photosensitive body, there is provided a f-θ lens. This is because the f-θ lens has an optical characteristic such as a corrective operation of a distortion aberration which guarantees a converging operation of the laser light and time linearity of a scan, thereby allowing the laser light having passed through the f-θ lens to be joint-scanned on the photosensitive body in a predetermine direction at an equal speed. However, due to a displacement of the characteristic of the f-θ lens, the laser light irradiated on the photosensitive body is sometimes displaced from an ideal image forming position. Hence, a frequency modulation technology is employed, in which this positional displacement of the image due to the f-θ lens characteristic is modulated to match a reference image clock so that the on/off timing of the laser light is subtly adjusted, thereby correcting the position of the dot formed on the photosensitive body (for example, Japanese Patent Application Laid-Open Patent No. H2-282763).
However, when the image clock is always constant, there are often the cases where a radiation noise is generated and the level of the radiation noise exceeds the value defined in the international radiation noise standard in the transmission path in which the on/off signal for turning on/off the laser light is transmitted from the generating circuit to a laser light driving circuit.
Further, when the frequency modulation technology is employed, though the radiation noise level is reduced, in the case where the f-θ lens having a characteristic to such an extent that there is no need for performing the frequency modulation is used, the image clock frequency becomes constant, and therefore, the radiation noise level becomes much severe.
Particularly, in a color image forming apparatus of a tandem system and the like in which the color drift in a main scan direction becomes a problem, the frequency modulation is often used for correcting the characteristic of the f-θ lens, while in the color image forming apparatus of one drum system in which there is no need for being very sensitive about the color drift in the main scan direction or a black and white image forming apparatus in which there is no need for taking into consideration the color drift, the frequency modulation is scarcely performed, and even in that case, there are often the cases where the radiation noise level exceeds the value of the international radiation noise standard, and this becomes a problem.